This invention relates to diodes such as diodes operating in the avalanche breakdown region.
Any reverse biased PN junction has a small reverse current flow due to the presence of holes and electrons in the vicinity of the depletion region. These holes and electrons are swept across the depletion region by the applied electric field and contribute to the leakage current of the PN junction. As the reverse bias on the PN junction is increased, the applied electric field increases and the carriers acquire increasing amounts of energy. At a predetermined critical electric field, many of the carriers traversing the depletion region acquire sufficient energy to create new hole-electron pairs upon colliding with lattice atoms. This is called the avalanche multiplication process and leads to a large and sudden increase in the reverse bias current since the newly created carriers are also capable of producing new hole-electron pairs upon colliding with other lattice atoms. It should be noted that for a large reverse current to flow by avalanche breakdown, two conditions must exist: first, there must be sufficient reverse bias potential across the PN junction to create a sufficient electric field in the depletion region to support avalanche multiplication which enables holes and electrons to accelerate across the PN junction and, thus, create many additional carriers. Second, there must be carriers available in the vicinity of the PN junction to begin and sustain the avalanche breakdown process.
PN junctions operating in the avalanche region, as described above, are widely used as voltage references and are typically called zener diodes, but are more correctly called avalanche breakdown diodes. However, one major problem with zener diodes is that they exhibit large levels of noise, or short term variations in the breakdown current, when biased around their breakdown voltage. One simplistic approach for reducing the noise due to avalanche breakdown of a PN junction is to place a capacitor across the PN junction. In general, as the value of the capacitor increases, the level of the noise decreases. However, a large capacitor is difficult to fabricate on an integrated circuit and typically reduces the speed of the circuit.
Another approach for reducing the noise due to avalanche breakdown of a PN junction is to bias the PN junction at a higher current. However, this approach consumes a large amount of power while only slightly reducing the noise.
Hence, there exists a need for providing a PN junction device that operates in the avalanche breakdown region while having reduced noise characteristics.